1. Field of the Invention
This invention relates generally to a variable-speed drive system for fans and pumps and more particularly to a combination variable-speed drive and speed reducer for a fan or a pump using a transmission that combines shaft inputs from a primary motor and an auxiliary motor/generator.
2. Discussion of the Background
The energy cost of fans and pumps amounts to tens of billions of dollars each year in the U.S. alone. Worldwide the cost probably exceeds one hundred billion dollars per year. Engineers and designers have devoted enormous effort in developing electrical and mechanical methods of varying capacity of fans and pumps, but most fans and pumps are still operated at constant flow.
Controlling the speed of fans and pumps is an important factor in reducing energy use in many applications. Fluid moving devices typically involve a cubic relationship between power and speed. At the same time flow is directly proportional to speed. These relationships mean that running a fan or pump at half speed gives half flow but uses only one eighth of the design input power. Other means of reducing flow such as throttling valves, inlet vanes, etc. give much smaller energy savings and usually have much higher noise levels than those found with variable speed fans and pumps.
In addition, it is frequently desirable to run a fan or pump at slow speeds, even at design conditions. For example, efficiency and sound levels decrease quite rapidly with the use of larger diameter, slower speed fans moving the same air flow. Common devices for providing slower speed include slow speed motors, which are costly and limited in availability. Belt drives are an inexpensive alternative, but require regular maintenance and reduced reliability. Gear drives are also sometimes used and give a higher reliability than belts but at an increased cost in many cases.
Several systems have been used for controlling the speed of fans and pumps. Among the more common are multispeed motors, variable frequency drives with induction motors, and magnetic clutches.
Epicyclic gears have been used as a part of variable-speed drive for a wide range of applications. Such devices have been described in U.S. Pat. Nos. 436,127; 711,663; 854,721; 855,623; 1,197,789; 1,235,132; 1,270,028; 1,409,061; 1,442,217; 1,500,860; 1,594,396; 1,828,944; 2,133,365; 2,384,776; 2,390,240; 2,390,487; 2,397,062; 2,436,936; 2,440,614; 2,485,126; 2,578,837; 2,581,886; 2,666,876; 2,693,080; 2,757,856; 3,405,572; 3,729,957; 3,853,432; 4,086,019; 4,441,462; 4,514,991; 4,467,230; 4,525,655; 4,579,019 and 4,916,894.
Drives for other applications have been developed that use a mechanical transmission to combine the input from two electric motors. Among the earliest mention of this approach is Edison U.S. Pat. No. 436,127 that describes a system for driving street cars.
Zinsmeyer, U.S. Pat. No. 4,514,991 describes a system for use with centrifugal compressors in a vapor compression system. This system is not suitable for fans and pumps since it is specifically designed to compress gas rather than move a fluid. It also is designed to increase the speed of the compressor compared to the motor input speed.